11)3/8????
1.)
there are four machines and it is known that exactly two of them are faulty. they are tested , one by one , in a random order till both the faulty machines are identified. then the probability that only two tests are needed is ?
2.) in a college of 300 students, every student reads 5 news papers and every news paper i read by 60 students. the number of news papers is ?
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7 Answers
131. 1/3
total no. of tests possible is 4C2 = 6 .... no. of cases in which only two tests are required is 2(getting 2 faulty machines in two tests...or getting the other 2)
Alternative:
getting faulty in 1st test 2/4
getting faulty in 2nd test 1/3
probab of this...1/6
getting working mach. in 1st test 2/4
getting working in 2nd 1/3
prbb of this1/6
req probab 1/6+1/6=1/3
12nd one i dun see any complexity... evry set of 60 students reads a set of 5 newspapers... so total no. of newspapers = 25...
1for d first one... total no. of tests possible is 4C2 = 6 .... n no. of cases in which only two tests are required is only 2...
so probability = 2/6 = 1/3
